The aim of this project is to determine the detailed molecular structure of the photosynthetic reaction center from the photosynthetic bacterium Rhodobacter sphaeroides by studying single crystals of this protein complex through x-ray diffraction. Reaction centers are intrinsic membrane proteins that carry out the initial steps in photosynthetic charge separations. The Rb. sphaeroides reaction center (Mr equals approximately 100,000) contains three protein subunits, four bacteriochlorophylls, two bacteriophenophytins, two ubiquinones, one carotenoid, and one nonheme iron. We will continue to refine the structure of the reaction center from R-26 and will compare it to other reaction center structures. We will start structure determinations on reaction centers that have been altered through site-specific mutagenesis. We plan to determine structures by using conventional crystallographic methods. The Rb. sphaeroides reaction center serves a model system to study how transmembrane electron transport is coupled to the production of membrane potential. This reaction center has been studied extensively through both spectroscopic and biochemical methods. The reaction center complex is functional in the crystal; therefore, we have a unique opportunity to correlate spectroscopic results with the three-dimensional structure of the complex. This study will contribute to the understanding of the structure of membrane proteins. The Rb. sphaeroides reaction center is only one of a few membrane proteins for which the availability of highly ordered crystals makes the determination of detailed structure possible. The correlation of its amino acid sequence with the three-dimensional structure will increase the database for membrane proteins, thereby markedly enhancing structure prediction efforts for these types of proteins.